Progressive multifocal lenses include two visual fields of different refractive powers and a visual field with progressively varying refractive power therebetween, in which the visual fields have no boundary therebetween, which has good appearance, and which provides visual fields of different refractive powers with a single lens. Accordingly, the progressive multifocal lenses are often used as eyeglass lenses having the power of correcting eyesight such as farsightedness. FIG. 11(a) shows a general structure of a conventional progressive multifocal lens which is commonly used as an eyeglass lens. The progressive multifocal lens 100 has a distance portion 11 serving as a visual field for viewing a far object at the upper part and a near portion 12 serving as a visual field with a power different from that of the distance portion 11 for viewing a near object below the distance portion 11. The distance portion 11 and the near portion 12 are smoothly communicated with each other through a progressive portion 13 serving as a visual field having a refractive power that varies continuously for viewing the intermediate object between a long distance and a short distance. As shown in FIG. 11(b), it is necessary for a single-plate lens used for eyeglasses to provide all functions required for eyeglasses such as vertex power that corresponds to the degree of user's eyes, cylindrical power for correcting astigmatism, addition power for correcting farsightedness, and prismatic power for correcting heterophoria with two refracting interfaces of an eyeball-side refracting interface 2 and a target object-side refracting interface 3. Accordingly, as shown in FIG. 11(b), in a conventional progressive multifocal lens 100a, a progressive refracting interface 14 having refractive power that varies continuously because of the distance portion 11, the near portion 12, and the progressive portion 13 is formed in the object-side refracting interface 3 and the eyeball-side refracting interface 2 is used as an astigmatism-correcting toric surface 15 etc. In contrast to the conventional progressive multifocal lens 100a, in order to provide a progressive multifocal lens in which the blurring or distortion of an image due to the variation in magnification of the distance portion 11 and the near portion 12 of the progressive multifocal lens is improved, the applicant has developed a so-called internal progressive multifocal lens 100b having a progressive refracting interface on the eyeball-side refracting interface 2 in view of the arrangement of the progressive refracting interface of the magnification factor of the progressive multifocal lens. The invention is described in WO 97/19382. The internal progressive multifocal lens 100b described in WO 97/19382 can reduce the difference in magnification between the distance portion and the near portion, thereby greatly reducing the resulting blurring and distortion. Since the internal progressive multifocal lens 100b has the progressive refracting interface 14 in the eyeball-side refracting interface 2, the astigmatism-correcting toric surface 15 can be combined to the eyeball-side refracting interface 2. The internal progressive multifocal lens 100b which has an eyesight corrective characteristic and an astigmatism corrective characteristic in the eyeball-side refracting interface 2 can reduce the difference in magnification between the distance portion and the near portion to a necessary minimum, having little image blurring and distortion, and being capable of correcting astigmatism, thereby providing a more comfortable visual field to users with astigmatism. The progressive multifocal lens 100b of which eyeball-side refracting interface 2 is given the eyesight corrective characteristic and the astigmatism corrective characteristic can be designed by a designing method including a first step of obtaining a progressive refracting interface (hereinafter, referred to as an original progressive refracting interface) only in order that the eyeball-side refracting interface exhibits a desired eyesight corrective characteristic, a second step of obtaining a toric surface (hereinafter, referred to as an original toric surface) only in order that the eyeball-side refracting interface exhibits a desired astigmatism corrective characteristic, and a third step of obtaining the eyeball-side refracting interface of the progressive multifocal lens from the original progressive refracting interface and the original toric surface. The above-described WO 97/19382 proposes a combination expression by which the original progressive refracting interface and the original toric surface are integrated with each other in the third step.
The progressive multifocal lens 100b in which the progressive refracting interface 14 and the toric surface 15 are combined in the eyeball-side refracting interface 2 with the combination expression proposed in WO 97/19382 is not subjected to interference with eyesight corrective power for correcting astigmatism almost all the region of a principal fixation line 16 and ensures astigmatism remarkably stably, as in the conventional progressive multifocal lens 100a. 
The toric surface is defined as a surface that has the maximum power in a certain meridian plane and the minimum power in a meridian plane perpendicular to that and has different curvature radii in planes intersecting at right angles. However, the meridian plane with the maximum power and the meridian plane with the minimum power are in circular arc shape in cross section and the shape of the curve therebetween is not particularly determined. Briefly, the kinds of the shape of the toric surface are limitless. Although the combination expression proposed in WO 97/19382 has the advantage of relatively easy calculation, it only combines the toric surface with the assumption that the curvature in the x-axis direction and the curvature in the y-axis direction are each fixed.
Accordingly, providing another combination expression that expresses the shape of the toric surface will provide a combined refracting interface composed of the progressive refracting interface and the toric surface which has more excellent astigmatism corrective power.
The present invention has been made in consideration of the above problems. Accordingly, an object of the invention is to provide a new progressive multifocal lens having eyesight corrective power and astigmatism corrective power in the same refracting interface by providing a new combination expression for combining a progressive refracting interface and a toric surface.
Another object of the invention is to provide a method for designing such a progressive multifocal lens.